Ready To Rumble: The Anatomy Of A Summer Thunderstorm

If you’re gazing skywards this summer and
notice menacing, cauliflower-like clouds billowing high, put
the washing on hold and bring the dog inside: a thunderstorm
might be on the way.

Thunderstorms can occur anywhere in
New Zealand at any time of the year. Arguably the most
impressive, however, are those that develop on a warm, humid
summer’s day, when heating of the land begins a process
that can transform a serene morning sky into a spectacular
tumult by mid-afternoon.

In New Zealand, the conditions
that favour summer thunderstorm formation occur most often
inland (where heating is strongest) and over high ground.
The North Island’s Central Plateau and ranges, the upper
Canterbury plains and the South Island high country are
prime breeding grounds. At other locations, like Auckland,
local sea breeze convergence plays a key role.

NIWA’s Dr Mike Revell has been studying the intricacies
of the atmosphere for nearly four decades. He says that
summer thunderstorms occur with deep cumulonimbus clouds
that typically extend from about a kilometre above the
surface, up through the troposphere (the lowest layer of the
atmosphere) to a height of 10 kilometres or so.

A number
of key ingredients are required for their
development.

A boost from
belowCumulonimbus clouds
grow in parcels of air rising from the Earth’s surface,
and an initial trigger to lift the air off the ground is
essential for their development. “Often the sun’s
heating of the land is enough,” Dr Revell says. “The
warmed land heats the air immediately in contact with it
and, as we learnt at school, warm air rises. This is known
as convection.

“There are other common lifting
mechanisms too, like breezes converging from different
directions. When that happens, there’s nowhere for the air
to go but up. An advancing cold front, or a breeze blowing
over a hill, might also start the process.”

Summer
thunderstorms in New Zealand are commonly triggered by a
combination of these mechanisms.

Let there be
clouds

A good supply of moisture at low levels is
also essential. As the parcel of air rises, it cools and
becomes less able to hold any moisture it contains. “If it
is sufficiently humid, the rising air will eventually cool
to saturation point, and clouds will form,” Dr Revell
says.

Higher still and
higher

Continued growth of those clouds into
active thunderstorms relies on ‘instability’ in the
troposphere. That means the parcel of lifted, saturated air
remains warmer (and hence less dense) than its surroundings,
so is able to keep on rising. This process is helped by
latent heat released as water droplets condense out of the
rising parcel of saturated air.

“In ‘stable’
conditions, the rising air will quickly run into a layer
that is of equal or higher temperature – known as an
‘inversion layer’ – putting a lid on the upwards
motion,” Dr Revell explains. “But if the air in the
middle and upper levels of the troposphere is unusually
cold, the parcel is more likely to remain warmer in
comparison, and keep rising higher and higher. That’s what
we mean by instability.

“Thunderstorms caused by
heating will often begin over the ranges or higher
ground,” he adds, “because the degree of heating is the
same as at lower elevations, but the hills are poking their
noses up into that colder air, so the heating is higher up
and instability is correspondingly
greater.”

Interestingly, some of the most vigorous
thunderstorm growth can occur when a thin inversion layer
lies underneath a deep layer of unstable air. Convection is
initially supressed by the low level inversion, allowing the
parcels of air below to become considerably warmer than the
air above the inversion. Eventually daytime heating will
make the convection strong enough for these parcels to burst
through the stable layer and then really strong updrafts
will occur.

Upwards growth is eventually suppressed by the
tropopause, an ever-present inversion layer marking the top
of the troposphere, at a height of nine to twelve kilometres
above New Zealand. Here the top of a mature cumulonimbus
cloud is forced to spread laterally, forming its classic
anvil shape.

Reaching flashpoint

“The convection process begins when the sun rises and
starts to heat the land surface,” says Dr Revell. “Then,
depending on atmospheric conditions, the thunderstorms are
ready to break by about mid-afternoon. They can last well
into the evening – normally as a result of new cells
continually replacing decaying cells, rather than a single,
long-lived storm.”

Lightning is an electrical
discharge that occurs in a thunderstorm. Lightning occurs
when an electrical charge is built up within a cloud, due to
static electricity generated by super-cooled water droplets
colliding with ice crystals near the freezing level. When a
large enough charge is built up, a large discharge will
occur and can be seen as lightning. Thunder is the shockwave
caused by the sudden expansion of a narrow channel of air,
as it is superheated by the lightning passing through
it.

Hail and tornadoes

Hail and
tornadoes are the spectacular – but often destructive –
accompaniments of very active thunderstorms.

For hail to
form, the cumulonimbus cloud must tower well above the
freezing level and contain an updraft strong enough to fling
water droplets quickly and repeatedly up into the frigid
air, adding a new layer of ice with every visit. Eventually
the hailstones become heavy enough to counteract the updraft
(which may be as strong as 100km/h) and fall to
Earth.

Tornado formation is complex and still only
partially understood. A very active thunderstorm with a
powerful updraft is required. Strong upward motion in a
confined area creates a vortex (in the same way that
bathwater pouring down a narrow plughole circulates rapidly)
which, under favourable conditions, narrows and accelerates
into the characteristic funnel of intensely circulating
wind.

The ‘Holy Grail’ of
forecasting

Pinpointing exactly where
thunderstorms will erupt, in a timeframe that is helpful to
people likely to be affected, remains a long-term goal for
the scientists at NIWA who study the weather.

“At the
moment, our computer models are very effective at
forecasting the conditions under which thunderstorms are
likely to develop over an area of several hundred square
kilometres,” says Dr Revell. “They can also model with
considerable accuracy the processes taking place within a
single cumulonimbus cell once the growth phase has begun.
But we remain some way off being able to forecast exactly
when and where, within the several hundred square kilometre
area, those individual cells will develop, to forewarn
people nearby of the potential impacts. Our modelling
resolution and capability are improving all the time – and
that remains the ‘Holy Grail’.”

In the meantime, Dr
Revell says, look to the summer skies for the tell-tale
signs of thunderstorm development. “It’s not often you
get to watch the complete cycle of wild weather evolving
from beginning to end. A summer thunderstorm is natural
theatre on a grand and spectacular
scale.”

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